Re: [PATCH 2/7] stacktrace,sched: Make stack_trace_save_tsk() more robust

From: Kees Cook
Date: Mon Oct 25 2021 - 16:52:12 EST


On Mon, Oct 25, 2021 at 10:38:33PM +0200, Peter Zijlstra wrote:
> On Fri, Oct 22, 2021 at 07:01:35PM +0200, Peter Zijlstra wrote:
> > On Fri, Oct 22, 2021 at 05:54:31PM +0100, Mark Rutland wrote:
> >
> > > > Pardon my thin understanding of the scheduler, but I assume this change
> > > > doesn't mean stack_trace_save_tsk() stops working for "current", right?
> > > > In trying to answer this for myself, I couldn't convince myself what value
> > > > current->__state have here. Is it one of TASK_(UN)INTERRUPTIBLE ?
> > >
> > > Regardless of that, current->on_rq will be non-zero, so you're right that this
> > > causes stack_trace_save_tsk() to not work for current, e.g.
> > >
> > > | # cat /proc/self/stack
> > > | # wc /proc/self/stack
> > > | 0 0 0 /proc/self/stack
> > >
> > > TBH, I think that (taking a step back from this issue in particular)
> > > stack_trace_save_tsk() *shouldn't* work for current, and callers *should* be
> > > forced to explicitly handle current separately from blocked tasks.
> >
> > That..
>
> So I think I'd prefer the following approach to that (and i'm not
> currently volunteering for it):
>
> - convert all archs to ARCH_STACKWALK; this gets the semantics out of
> arch code and into the single kernel/stacktrace.c file.
>
> - bike-shed a new/improved stack_trace_save*() API and implement it
> *once* in generic code based on arch_stack_walk().
>
> - convert users; delete old etc..
>
> For now, current users of stack_trace_save_tsk() very much expect
> tsk==current to work.
>
> > > So we could fix this in the stacktrace code with:
> > >
> > > | diff --git a/kernel/stacktrace.c b/kernel/stacktrace.c
> > > | index a1cdbf8c3ef8..327af9ff2c55 100644
> > > | --- a/kernel/stacktrace.c
> > > | +++ b/kernel/stacktrace.c
> > > | @@ -149,7 +149,10 @@ unsigned int stack_trace_save_tsk(struct task_struct *tsk, unsigned long *store,
> > > | .skip = skipnr + (current == tsk),
> > > | };
> > > |
> > > | - task_try_func(tsk, try_arch_stack_walk_tsk, &c);
> > > | + if (tsk == current)
> > > | + try_arch_stack_walk_tsk(tsk, &c);
> > > | + else
> > > | + task_try_func(tsk, try_arch_stack_walk_tsk, &c);
> > > |
> > > | return c.len;
> > > | }
> > >
> > > ... and we could rename task_try_func() to blocked_task_try_func(), and
> > > later push the distinction into higher-level callers.
> >
> > I think I favour this fix if we have to. But that's for next week :-)
>
> I ended up with the below delta to this patch.
>
> --- a/kernel/stacktrace.c
> +++ b/kernel/stacktrace.c
> @@ -101,7 +101,7 @@ static bool stack_trace_consume_entry_no
> }
>
> /**
> - * stack_trace_save - Save a stack trace into a storage array
> + * stack_trace_save - Save a stack trace (of current) into a storage array
> * @store: Pointer to storage array
> * @size: Size of the storage array
> * @skipnr: Number of entries to skip at the start of the stack trace
> @@ -132,7 +132,7 @@ static int try_arch_stack_walk_tsk(struc
>
> /**
> * stack_trace_save_tsk - Save a task stack trace into a storage array
> - * @task: The task to examine
> + * @task: The task to examine (current allowed)
> * @store: Pointer to storage array
> * @size: Size of the storage array
> * @skipnr: Number of entries to skip at the start of the stack trace
> @@ -149,13 +149,25 @@ unsigned int stack_trace_save_tsk(struct
> .skip = skipnr + (current == tsk),
> };
>
> - task_try_func(tsk, try_arch_stack_walk_tsk, &c);
> + /*
> + * If the task doesn't have a stack (e.g., a zombie), the stack is
> + * empty.
> + */
> + if (!try_get_task_stack(tsk))
> + return 0;
> +
> + if (tsk == current)
> + try_arch_stack_walk_tsk(tsk, &c);
> + else
> + task_try_func(tsk, try_arch_stack_walk_tsk, &c);
> +
> + put_task_stack(tsk);
>
> return c.len;
> }
>
> /**
> - * stack_trace_save_regs - Save a stack trace based on pt_regs into a storage array
> + * stack_trace_save_regs - Save a stack trace (of current) based on pt_regs into a storage array
> * @regs: Pointer to pt_regs to examine
> * @store: Pointer to storage array
> * @size: Size of the storage array

Looks good to me, though I did like Mark's idea to name "task_try_func"
to "task_blocked_try_func" or something like that to make the "why can
this fail?" be more self-documenting. *shrug*

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

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Kees Cook